The growth and consumption of kleingrass (Panicum coloratum L.) by growing beef steers : a model

Abstract

Of all the problems that confront agriculture perhaps the unpredictability of weather is the most difficult to quantify. Twenty years of climatalogical data were obtained for Beeville, Texas and cumulative frequency distributions were fit to maximum and minimum monthly temperatures and for monthly precipitation. A mathematical simulation model, stochastically then determined the climatalogical input to a soil moisture model. The soil moisture model was then the basis for a kleingrass growth model. Utilizing different management schemes, different stocking rates, and different beef cattle energy systems, a simulation model that permitted beef steers to graze the available kleingrass was depicted. The model included such seemingly diverse inputs as stochastic climatological input, water retention curves at different soil depths, potential evapotranspiration, forage growth, forage intake by steers and steer performance on the available forage and of necessity the model had to amalgamate many concepts from many different disciplines. Modeling such a complex interaction of variables permitted the elucidation of a number of areas that still require further research: forage growth in relation to available water, the rooting characteristics of the various forage species, forage consumption and fecal output of steers in relation to grazing pressure and the validity of currently available energy systems to assess the performance of grazing beef cattle. In general the climatological output of the model compared well to the historical weather patterns at Beeville (P < .05). Simulated animal performance compared favorably to historical data of steers grazing kleingrass at Beeville and the corresponding difficulties of model validation were noted. Light stocking rates resulted in heavier (P < .05) cumulative weight gains than heavier stocking rates when steers were removed from forage. Management systems that judiciously introduce and remove steers from grazing produced more cumulative weight gains (P < .05) per ha per season than systems that permitted steers to graze when forage was becoming quickly unavailable. Heavier stocking rates on the average produced more (P < .05) cumulative weight per ha for the grazing season than lighter stocking rates, however the variability of the cumulative weight gains was much higher for the heavier stocking rates than for the lighter stocking rates.